What are you using for the main power source?
Have you checked the board voltages?
The original i-o wall transformer was rated at 19 volts, 1.8 amps.
You can check for 19 volts between inductor L33 and ground. L33 is beside the bios chip.
You can check for 5 volts between pin 10 of CN12 and ground. CN12 is the serial port beside the memory.
It is possible that your power supply is failing when it gets warm and can't supply enough voltage.
How did you do a core voltage mod on a V1? You could remove your modification and try booting with the original cpu.